1. Field of the Invention
The present invention relates to a display device having a single pixel made up of subpixels of RGBW (red, green, blue and white), that displays input RGB data by converting to R′G′B′W data.
2. Description of the Related Art
FIG. 1 shows one example of dot layout of a matrix type organic EL (OLED) panel having single pixels made up of three subpixels (dots) of ordinary red green and blue (R, G, B), and FIG. 2 shows dot layout of a matrix type EL panel that also uses white (W) in addition to R, G, B. In FIG. 2, RGBW are arranged in the horizontal direction, while in FIG. 3 RGBW are arranged grouped together in pixels of 2×2.
The RGBW type is intended to improve brightness and power reduction of a panel by using W bits that have higher light emitting efficiency than R, G, B. As a method of implementing an RGBW type panel, there is a method using organic EL elements that emit light of respective colors in each dot, and a method of implementing dots other than W by overlaying optical filters of red, green and blue on a white organic EL element.
FIG. 4 is a CIE 1931 color space chromaticity diagram, and shows one example of chromaticity of white (W) that uses a white pixel in addition to the normal primary colors of red, green and blue (R, G, B). This chromaticity of W does not always need to match a reference white color of a display.
FIG. 5 shows a method of converting RGB input signals, that can display reference white color of a display when R=1, G=1 and B=1, to RGBW pixel signals.
First, in the case where the emitted color of a W dot does not match the reference white color of the display, the following calculation is carried out on the input RGB signals to perform normalization to the emission color of the W dot (S11).
                    Equation        ⁢                                  ⁢        1.                                                                      [                                                    Rn                                                                    Gn                                                                    Bn                                              ]                =                              [                                                            a                                                  0                                                  0                                                                              0                                                  b                                                  0                                                                              0                                                  0                                                  c                                                      ]                    ×                      [                                                            R                                                                              G                                                                              B                                                      ]                                              expression        ⁢                                  ⁢        1            
Here, R, G, B are input signals, Rn, Gn, Bn are red, green and blue signals that have been normalized, and a, b, c are coefficients that have been selected so that when R=1/a, G=1/b and B=1/c, respectively, they have the same brightness and chromaticity as W=1.
As an example of arithmetic expressions for the most fundamental S, F2, F3, the following can be considered:S=min(Rn,Gn,Bn)  expression 2F2(S)=−S  expression 3F3(S)=S  expression 4
In this case, for (Rn, Gn, Bn) obtained in S11, in step S12 S (minimum values within RGB components that have been normalized) is calculated from expression 2 (S12), and Rn′, Gn′, Bn′ are obtained by subtracting the obtained S from Rn, Gn, Bn (S13, S14). Also, S is output directly as a white value (Wh) (S15).
In this case, it will be understood that as the color of a pixel to be displayed approaches an achromatic color, the proportion of a W dot that is lit up is increased. The power consumption of a panel therefore decreases as the proportion of color approaching an achromatic color increases within a displayed image, compared to when using RGB only.
Also, similarly to normalization to the emission color of a W dot, when the emission color of a W dot does not match reference white of the display, final normalization to reference white is carried out (S16). This final normalization to reference white is carried out as follows.
                    Equation        ⁢                                  ⁢        2.                                                                      [                                                                      R                  ′                                                                                                      G                  ′                                                                                                      B                  ′                                                              ]                =                              [                                                                                1                    /                    a                                                                    0                                                  0                                                                              0                                                                      1                    /                    b                                                                    0                                                                              0                                                  0                                                                      1                    /                    c                                                                        ]                    ×                      [                                                            Rn                                                                              Gn                                                                              Bn                                                      ]                                              expression        ⁢                                  ⁢        5            
Normally, there are few images made up of only pure colors, and there are hardly any cases where W dots are used, which means that overall power consumption is lowered on average compared to when using only RGB pixels.
Also, in the case where M is defined as 0≦M≦1, and the following expressions used in F2 and F3, usage rate of the W dots will vary depending on the value of M.F2(S)=−MS  expression 6F3(S)=MS  expression 7
From the point of view of power consumption, it is best if M=1, that is, if usage efficiency is 100%. However, from the point of view of visual resolution it is better to select a value of M such that all of RGBW are lit to the greatest extent possible (refer to patent publication 1).
FIG. 6 is a schematic drawing of a conversion method when normalization is not carried out. For an input signal, minimum values S within RGB are obtained (S21), and the obtained values S are multiplied by a coefficient M to determine white (Wh) (S22). Together with outputting this Wh, it is subtracted from respective RGB components (S23) to obtain finally converted R′, G′, B′.